1. Field of the Invention
This invention relates to a luminance adjusting system for a flat matrix type cathode-ray tube(CRT), which controls the brightness of a picture displayed on the CRT according to environmental brightness.
2. Description of the Prior Art
An example of a conventional flat matrix type CRT is so designed that grid electrodes are made up of a plurality of scanning electrodes and a plurality of data electrodes which are arranged to form a matrix structure together with the scanning electrodes, and pulse voltages of predetermined gradations are applied selectively to selected ones of the electrodes in the two different groups to cause the picture elements corresponding to the selected electrodes to emit light.
In this operation, as is well known in the art, electrons emitted from a direct-heating type linear filament are accelerated by the voltages applied to the grid electrodes consisting of the scanning electrodes and the data electrodes, to impinge an anode at a high voltage, whereby a light emitting material such as a fluorescent layer which is formed on the anode by coating, is caused to emit light by the energy of collision.
FIG. 1 shows the grid electrode structure of the flat matrix type CRT. In the flat matrix type CRT, the grid electrode structure is a 4.times.4 matrix structure consisting of scanning electrodes X1 through X4 and data electrodes Y1 through Y4. In correspondence to the 4x4 matrix structure, fluorescent substances of red, green and blue are applied to the predetermined parts of the anode surface (not shown) as required, so that sixteen (16) picture elements in one element can appear red (R), green(G), and blue (B).
A desired one of the picture elements can be selected by applying voltages to the scanning electrode and the data electrode which are provided for the picture element. And only the picture element thus selected can emit light.
FIGS. 2(a) through 2(e) are diagrams for a description of the display system in which, in the halftone display by the above-described conventional CRT, one field is divided into four parts, and, in a 1/4 duty, display is carried out with 64 gradations. FIGS. 2(a) through 2(d), reference characters GX.sub.1 through GX.sub.4 designate the voltage waveforms of scanning signals applied to the scanning electrodes X1 through X4 shown in FIG. 1, respectively. In FIG. 3(e), reference character GY designates the pulse-width-controlled voltage waveform of one of data signals GY.sub.1 through GY.sub.4 respectively applied to the data electrodes Y.sub.1 through Y.sub.4 shown in FIG. 1. When the scanning signal and the data signal are both at the high level, the corresponding picture element is caused to emit light. In FIG. 2(e), reference character T.sub.B designates a blanking time.
In the above-described conventional CRT, of the video signal to be displayed, only the parts corresponding to brightness instructions are subjected to light emission control. That is, only the degrees of brightness on the display instruction are adjusted by the pulse width control of the data signals. Accordingly, for the CRT, it is impossible to perform the adjustment of the brightness of a display picture according to the environmental brightness. Therefore, depending on the environmental brightness, the display picture becomes difficult to observe.